1. Field of the Invention
The present invention relates to the field of fibers and products made from fibers, specifically to a novel method for the production of natural cellulosic fiber bundles from stalks (stems or straws), leaves, and husks of cellulose sources such as corn, switchgrass, sorghum, rice, wheat, soybean, cotton, and barley that are suitable for, among other things, textile applications, non-woven mats and filters, and fiber reinforced composites.
2. Description of the Related Technology
Cellulose is the most abundantly available organic matter on earth. Cellulose in its natural and regenerated form is a major source of fiber for textile applications. Globally, farming generates millions of tons of agricultural byproducts each year. Some of the byproducts are used as animal feed and for other small scale applications. Many of the agricultural byproducts contain substantial amounts of cellulose, especially in fibrous form. Utilizing agricultural byproducts more fully would benefit growers economically and provide environmental benefits by reducing the amount of byproduct disposal. Yet another environmental benefit is that products made using these agricultural byproducts may be made 100% bio-degradable (See, e.g., Chen et al., U.S. Pat. No. 6,083,582).
Natural cellulosic fibers, or fiber bundles, are derived from various parts of plants. The fibers are mainly classified as seed fibers (e.g., from cotton and kapok), stem or bast fibers (e.g., from flax, jute, hemp, kenaf, and sugarcane), and leaf fibers (e.g., from pineapple, banana). These fibers may be from plants grown primarily for the fibers (e.g., cotton, flax, hemp, kenaf, etc.) or from plants in which the fibers are primarily considered a byproduct such as coconut (the fibers are often referred to as “coir”), sugarcane, banana, and pineapple. The byproduct-type fibers have not been used extensively for several reasons including limited availability, difficulty in extraction, lesser performance-related properties, and limited growing regions. A general classification of natural textile fibers known to those of ordinary skill in the art is provided in Table A below. Conspicuously missing from the listed subgroups of cellulose fiber classifications are “husk fibers” despite the fact that cornhusk is readily available throughout a wide range of geographic regions. Also missing from the listed subgroups of cellulose fiber classifications are other common biomass sources such as switchgrass, sorghum, and rice straw.
TABLE AClassification of Natural Textile Fibers*CelluloseProteinMineralStem (Bast)Staple-hairAsbestosflax, hemp, jutesheep, goat, alpaca,chrysotilekenaf, ramie, linencamel, cashmere, IlamacrocidoliteFruitmohair, rabbit, vicunashell of coconut (coir)FilamentLeafsilkabaca/manila sisalSeedcotton, kapok*http:/www/nexiaonline.com/indusprt/classification.asp
Textile fibers from the stalks or stems and leaves of plants are conventionally extracted by a process known as “retting.” A traditional retting process is dew retting, which utilizes bacteria and fungi in the environment to delignify the fibers to a state suitable for processing using conventional textile machinery. Dew retting, however, tends to be inconsistent, tends to result in poor fiber quality, usually can only be performed in limited geographical regions, and occupies agricultural land during the retting process (see, e.g., Akin et al., Textiles Research Journal 69 (10), 747-753 (1999)). Chemical methods for retting have been used and these typically produce fibers that are more consistent and with improved physical properties compared to dew-retted fibers. Typical chemical-retting methods use alkalies in combination with other chemicals. Due to the environmental and waste disposal concerns, however, alternatives to chemical retting are being investigated (see, e.g., Wang et al., Textile Research Journal 73(4), 339-344 (2003)). For example, enzymatic retting is being researched as a more ecological-friendly process for fiber extraction. Cost, quality of fibers obtained, and difficulty in controlling the process have, thusfar, limited the use of enzyme retting in commercial scale applications.
Cornhusk contains about 40% cellulose, about 45% hemicellulose, about 7% lignin, about 2% protein and about 3% ash (see, e.g., Branka et al., Journal of Agricultural Food Chemistry, 34, 1019-1024 (1986)). Cellulose fibers in cornhusk are interconnected with each other to form large bundles that are hundreds of micrometers to millimeters wide. These large bundles are connected to each other by films. The cellulose fibers and fiber bundles are connected, primarily, by lignin and hemicellulose. The ultimate fibers in cornhusk are about 0.5 to about 1 mm in length. These ultimate fibers, which may also be referred to as single fibers or individual cells, are considered to be too short, too weak, or both for textile applications. In general, for a fiber to be considered suitable for textile use, among other things, it preferably has a length that is at least about 1,000 times its diameter. For practical purposes, it is preferred that the fiber length is at least about 1.0 cm. Additionally, a fiber preferably has an adequate tensile strength such as at least about 1 gram per denier. Denier is the common term used to describe the fineness or linear density of a textile material that is quantified as the materials weight in grams per 9,000 meters of that material.
Although the above-described retting processes have been used to produce natural cellulosic fibers from stalks or stems and leaves of plants, to date there has been no process for treating cellulose fiber sources such as cornhusks, cornstalks, cotton stalks, switchgrass stems and leaves, sorghum stems and leaves, rice straw, wheat straw, barley straw, and soybean straw to yield natural cellulosic fibers that are suitable for textile use. More specifically, using known methods for treating bast and leaf fibers to treat cellulose fiber sources such as cornhusks, rice straw, switchgrass, and sorghum results in fibers or fiber bundles that are too coarse, too small, too weak, or a combination of negative attributes that prevent their use in applications such as textiles and fiber reinforced composites. Accordingly, there is a need for a method of processing cellulose fiber sources such as cornhusks, rice straw, switchgrass, and sorghum to yield natural cellulosic fiber bundles that are suitable for textile and other industrial applications.